3 replies to this topic

[process chemical]

HI,
Presently I am designing a turrert moored FPSO.The well fluid arrival pressure is 9.5 barg at well head.FPSO is located 2.5 km away from wellhead.After performing hydraulic anaysis for 2.5km pipeline, i am getting 3.86 barg at FPSO riser hangoff. Available pressure for the first stage separator (HP separator) at FPSO topside is 1.8 barg considering swivel & piping pressure drop. will there be any concerns to operate the first stage separator at this pressure and what will be impact?

Thanks in advance

[paulhorth]

Process,

That is a low pressure for a HP Separator. Presumably you also have a LP Separator running at an even lower pressure? Why not use just one?

One of the purposes of the separator system ( one or two) is to achieve a maximum vapour pressure spec for the crude, usually 10 psi RVP. I assume that you have done the simulation which demonstrates that with your operating pressures, you will meet the required spec. If the oil is too cold, then you will not make this spec even at atmospheric pressure in the LP Separator, and some heating might be required.

The pressure in the HP separator has to be high enough to get the oil into the next vessel,and also to get the produced water out through whatever system you have. You can use pumps of course but they will need some height for the NPSH.
There can be a major impact on the gas compression if the separator pressure is lower than originally designed for.

Have you checked that you have allowed sufficient static head from the sea bed to the FPSO? (that would make the pressure even lower...)

Paul

[process chemical]

Dear paul,
Thanks for your reply.
I have considered the static height from seabed to FPSO during hydraulic study.
I have added an well fluid inlet heater before the separator to meet the RVP of the crude oil.
will there be any concern in designing the separator withrespect to the inlet device and meeting of carryover specification of gas at this low operating pressure of 1.8 barg?

[paulhorth]

Process,

I don't think that the low pressure will give rise to any concern about the gas-liquid separation performance. After all, flare drums are typically designed to separate droplets at close to atmspheric pressure. So long as you size the vessel according to the criteria for gas velocity given in many separation design sources you should be OK. You should consider a droplet separation device such as a mist mat or a vane pack at the gas outlet, this will allow a higher horizontal gas velocity and thus a smaller vessel diameter.

Paul